Acid rain is a widespread term used to describe all forms of acid precipitation (rain, snow, hail, fog, etc.). Atmospheric pollutants, particularly oxides of sulphur and nitrogen, can cause precipitation to become more acidic when converted to sulphuric and nitric acids, hence the term acid rain. Acid deposition, acid rain and acid precipitation all relate to the chemistry of air pollution and moisture in the atmosphere. Scientists generally use the term acid deposition but all three terms relate to the same issue.

The term acid rain was first used by Robert Angus Smith, a scientist working in Manchester in the 1870s. The problem of acid rain is hence not a new one but the nature of the problem has changed from being a local problem for towns and cities to being an international problem. In Smith’s time, acid rain fell both in towns and cities whilst today pollutants can be transported thousands of kilometres due to the introduction of tall chimneys dispersing pollutants high into the atmosphere.

Precipitation is naturally acidic because of carbon dioxide in the atmosphere. The burning of fossil fuels (coal, oil and gas) produces sulphur dioxide and nitrogen oxides which can increase the acidity of rain or other precipitation. Sources of sulphur dioxide and oxides of nitrogen may be natural such as volcanoes, oceans, biological decay and forest fires, or may arise from combustion sources. The increasing demand for electricity and the rise in the number of motor vehicles in recent decades has meant that emissions of acidifying pollutants have increased dramatically from human sources, particularly since the 1950s. Emissions of such pollutants are heavily concentrated in the northern hemisphere, especially in Europe and North America. As a result, precipitation is generally acidic in these countries.

In the 1970s and 1980s, Scandinavian countries began to notice the effects of acid deposition on trees and freshwaters. Much of the pollution causing this damage was identified as being transported from other more polluting countries. Acid rain became an international concern.

The pH scale is used to measure the acidity of acid rain which is determined by the hydrogen ion content (H+). This scale was invented by a Danish scientist called Sorenson in 1909. The pH scale ranges from 0, which is strongly acid, to 14 which is strongly alkaline, the scale point 7 being neutral. The pH scale is logarithmic rather than linear, so there is a ten-fold increase in acidity with each pH unit, such that rainfall with pH 5 is ten times more acidic than pH 6, rainfall with pH 4 is 100 times more acidic than pH 6 and rainfall with pH 3 is 1000 times more acidic than pH 6.

Acid rain became particularly prominent as a media issue during the 1980s. However, during the 1970s many countries started to notice changes in fish populations in lakes and damage to certain trees. By the late 1970s concern led to international efforts to identify the causes and effects of long-range (transboundary) transport of air pollutants, and thus during the 1980s much research was conducted in Europe and North America. International legislation during the 1980s and 1990s has led to reductions in sulphur dioxide emissions in many countries but reductions in emissions of nitrogen oxides have been much less.

Although media attention has shifted towards other environmental issues such as global warming, acid rain continues to be a problem at the beginning of the 21st century